Camille Lobry

Tet2 Loss Leads to Increased Hematopoietic Stem Cell Self-Renewal and Myeloid Transformation

Somatic loss-of-function mutations in the ten-eleven translocation 2 (TET2) gene occur in a significant proportion of patients with myeloid malignancies. Although there are extensive genetic data implicating TET2 mutations in myeloid transformation, the consequences of Tet2 loss in hematopoietic development have not been delineated. We report here an animal model of conditional Tet2 loss in the hematopoietic compartment that leads to increased stem cell self-renewal in vivo as assessed by competitive transplant assays. Tet2 loss leads to a progressive enlargement of the hematopoietic stem cell compartment and eventual myeloproliferation in vivo, including splenomegaly, monocytosis, and extramedullary hematopoiesis. In addition, Tet2(+/-) mice also displayed increased stem cell self-renewal and extramedullary hematopoiesis, suggesting that Tet2 haploinsufficiency contributes to hematopoietic transformation in vivo.

A novel tumour-suppressor function for the Notch pathway in myeloid leukaemia

Notch signalling is a central regulator of differentiation in a variety of organisms and tissue types. Its activity is controlled by the multi-subunit γ-secretase (γSE) complex. Although Notch signalling can play both oncogenic and tumour-suppressor roles in solid tumours, in the haematopoietic system it is exclusively oncogenic, notably in T-cell acute lymphoblastic leukaemia, a disease characterized by Notch1-activating mutations. Here we identify novel somatic-inactivating Notch pathway mutations in a fraction of patients with chronic myelomonocytic leukaemia (CMML). Inactivation of Notch signalling in mouse haematopoietic stem cells (HSCs) results in an aberrant accumulation of granulocyte/monocyte progenitors (GMPs), extramedullary haematopoieisis and the induction of CMML-like disease. Transcriptome analysis revealed that Notch signalling regulates an extensive myelomonocytic-specific gene signature, through the direct suppression of gene transcription by the Notch target Hes1. Our studies identify a novel role for Notch signalling during early haematopoietic stem cell differentiation and suggest that the Notch pathway can play both tumour-promoting and -suppressive roles within the same tissue.

Oncogenic and tumor suppressor functions of Notch in cancer: it’s NOTCH what you think

Aifantis and colleagues examine the conflicting roles of Notch signaling in various cancer types.

SRSF2 Mutations Contribute to Myelodysplasia by Mutant-Specific Effects on Exon Recognition

Mutations affecting spliceosomal proteins are the most common mutations in patients with myelodysplastic syndromes (MDS), but their role in MDS pathogenesis has not been delineated. Here we report that mutations affecting the splicing factor SRSF2 directly impair hematopoietic differentiation in vivo, which is not due to SRSF2 loss of function. By contrast, SRSF2 mutations alter SRSF2s normal sequence-specific RNA binding activity, thereby altering the recognition of specific exonic splicing enhancer motifs to drive recurrent mis-splicing of key hematopoietic regulators. This includes SRSF2 mutation-dependent splicing of EZH2, which triggers nonsense-mediated decay, which, in turn, results in impaired hematopoietic differentiation. These data provide a mechanistic link between a mutant spliceosomal protein, alterations in the splicing of key regulators, and impaired hematopoiesis.

Distinct TCR signaling pathways drive proliferation and cytokine production in T cells

The physiological basis and mechanistic requirements for a large number of functional immunoreceptor tyrosine-based activation motifs (ITAMs; high ITAM multiplicity) in the complex of the T cell antigen receptor (TCR) and the invariant signaling protein CD3 remain obscure. Here we found that whereas a low multiplicity of TCR-CD3 ITAMs was sufficient to engage canonical TCR-induced signaling events that led to cytokine secretion, a high multiplicity of TCR-CD3 ITAMs was required for TCR-driven proliferation. This was dependent on the formation of compact immunological synapses, interaction of the adaptor Vav1 with phosphorylated CD3 ITAMs to mediate the recruitment and activation of the oncogenic transcription factor Notch1 and, ultimately, proliferation induced by the cell-cycle regulator c-Myc. Analogous mechanistic events were also needed to drive proliferation in response to weak peptide agonists. Thus, the TCR-driven pathways that initiate cytokine secretion and proliferation are separable and are coordinated by the multiplicity of phosphorylated ITAMs in TCR-CD3.

T-cell factor 1 is a gatekeeper for T-cell specification in response to Notch signaling

Although transcriptional programs associated with T-cell specification and commitment have been described, the functional hierarchy and the roles of key regulators in structuring/orchestrating these programs remain unclear. Activation of Notch signaling in uncommitted precursors by the thymic stroma initiates the T-cell differentiation program. One regulator first induced in these precursors is the DNA-binding protein T-cell factor 1 (Tcf-1), a T-cell–specific mediator of Wnt signaling. However, the specific contribution of Tcf-1 to early T-cell development and the signals inducing it in these cells remain unclear. Here we assign functional significance to Tcf-1 as a gatekeeper of T-cell fate and show that Tcf-1 is directly activated by Notch signals. Tcf-1 is required at the earliest phase of T-cell determination for progression beyond the early thymic progenitor stage. The global expression profile of Tcf-1–deficient progenitors indicates that basic processes of DNA metabolism are down-regulated in its absence, and the blocked T-cell progenitors become abortive and die by apoptosis. Our data thus add an important functional relationship to the roadmap of T-cell development.

Notch signaling: switching an oncogene to a tumor suppressor

The Notch signaling pathway is a regulator of self-renewal and differentiation in several tissues and cell types. Notch is a binary cell-fate determinant, and its hyperactivation has been implicated as oncogenic in several cancers including breast cancer and T-cell acute lymphoblastic leukemia (T-ALL). Recently, several studies also unraveled tumor-suppressor roles for Notch signaling in different tissues, including tissues where it was before recognized as an oncogene in specific lineages. Whereas involvement of Notch as an oncogene in several lymphoid malignancies (T-ALL, B-chronic lymphocytic leukemia, splenic marginal zone lymphoma) is well characterized, there is growing evidence involving Notch signaling as a tumor suppressor in myeloid malignancies. It therefore appears that Notch signaling pathways oncogenic or tumor-suppressor abilities are highly context dependent. In this review, we summarize and discuss latest advances in the understanding of this dual role in hematopoiesis and the possible consequences for the treatment of hematologic malignancies.

Modulation of splicing catalysis for therapeutic targeting of leukemia with mutations in genes encoding spliceosomal proteins.

Mutations in genes encoding splicing factors (which we refer to as spliceosomal genes) are commonly found in patients with myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). These mutations recurrently affect specific amino acid residues, leading to perturbed normal splice site and exon recognition. Spliceosomal gene mutations are always heterozygous and rarely occur together with one another, suggesting that cells may tolerate only a partial deviation from normal splicing activity. To test this hypothesis, we engineered mice to express a mutated allele of serine/arginine-rich splicing factor 2 (Srsf2P95H)—which commonly occurs in individuals with MDS and AML—in an inducible, hemizygous manner in hematopoietic cells. These mice rapidly succumbed to fatal bone marrow failure, demonstrating that Srsf2-mutated cells depend on the wild-type Srsf2 allele for survival. In the context of leukemia, treatment with the spliceosome inhibitor E7107 (refs. 7,8) resulted in substantial reductions in leukemic burden, specifically in isogenic mouse leukemias and patient-derived xenograft AMLs carrying spliceosomal mutations. Whereas E7107 treatment of mice resulted in widespread intron retention and cassette exon skipping in leukemic cells regardless of Srsf2 genotype, the magnitude of splicing inhibition following E7107 treatment was greater in Srsf2-mutated than in Srsf2-wild-type leukemia, consistent with the differential effect of E7107 on survival. Collectively, these data provide genetic and pharmacologic evidence that leukemias with spliceosomal gene mutations are preferentially susceptible to additional splicing perturbations in vivo as compared to leukemias without such mutations. Modulation of spliceosome function may thus provide a new therapeutic avenue in genetically defined subsets of individuals with MDS or AML.

Notch pathway activation targets AML-initiating cell homeostasis and differentiation

Notch behaves as a tumor suppressor in AML, and Notch activation induces cell cycle arrest, differentiation, and apoptosis of AML-initiating cells.

PHF6 mutations in adult acute myeloid leukemia

Loss of function mutations and deletions encompassing the plant homeodomain finger 6 (PHF6) gene are present in about 20% of T-cell acute lymphoblastic leukemias (ALLs). Here, we report the identification of recurrent mutations in PHF6 in 10/353 adult acute myeloid leukemias (AMLs). Genetic lesions in PHF6 found in AMLs are frameshift and nonsense mutations distributed through the gene or point mutations involving the second plant homeodomain (PHD)-like domain of the protein. As in the case of T-ALL, where PHF6 alterations are found almost exclusively in males, mutations in PHF6 were seven times more prevalent in males than in females with AML. Overall, these results identify PHF6 as a tumor suppressor gene mutated in AML and extend the role of this X-linked tumor suppressor gene in the pathogenesis of hematologic tumors.